The present invention relates to a level shift circuit and more particularly to a small signal level shift circuit for shifting the level of a small signal without distortion.
Generally, in circuits or devices for processing a signal generated from a signal source such as a sensor, a monitoring circuit, a signal generating circuit and a level sensing circuit, etc., the levels of the generated signals required in the above circuits are different. A signal level is shifted by a level required in a level shift circuit shown in FIG. 1. In FIG. 1, the collector terminal of transistor 2 is coupled to a power source voltage Vcc, its base terminal is coupled to a signal Vs generated from a signal source, and its emitter terminal is grounded through serially connected resistors 4 and 6, thereby obtaining an output signal Vo at a node between the resistors 4 and 6. If a signal Vs such as FIG. 2A is supplied to the base terminal of the transistor 2, the transistor 2 amplifies the signal Vs. At this time, at the node between resistors 4 and 6, a level shifted signal Vo such as FIG. 2B is generated by adding the amplified signal to a signal level obtained by dividing the power source voltage Vcc. Accordingly, the signal Vs is shifted by a predetermined voltage level V.sub.SET.
As described above, the conventional level shift circuit generates the following problems according to the use of transistor. First, when a signal level is shifted largely, a transistor having a large small-signal current amplification rate h.sub.fe should be used. But, in this case, noise level is also increased, thereby generating a false operation, so that an extra circuit for preventing the false operation is needed. Secondly, when the signal level is very low, the transistor is in a cut-off region, so that the transistor does not operate. Thirdly, distortion caused by several operation characteristics of transistor is generated.
Meanwhile, in a switching mode power supply (hereinafter referred to as SMPS) widely used as power supply of several electric or electronic devices, signal transfer is needed, which is described in detail as follows. With reference to FIG. 3 showing an example of multi-output SMPS, a schematic operation useful for understanding of the present invention is described. An input rectifying circuit 12 rectifies an AC (alternating current) power source through an AC input power source 10 and supplies the rectified power source to a transformer 14 and a switching control unit 16. Generally, as the switching control unit 16, one-chip IC such as UC1842 which is a PWM controller of UNITRODE company in U.S.A. is used. The switching control, unit 16 is operated by the DC (direct current) power source supplied from the input rectifying circuit 12, thereby generating a pulse width modulation (hereinafter referred to as PWM) signal having a predetermined frequency. A switching circuit 18 coupled between a primary side of the transformer 14 and the switching control unit 16 induces power source at a secondary side of the transformer 14, by switching the DC power source supplied to the primary side of the transformer 14 in response to the PWM signal. The induced power source is rectified and smoothed in first and second output rectifying circuits 22 and 24, and then is supplied to a load through first and second DC output power sources 26 and 28, respectively. An insulating circuit 30 and a feedback circuit 32 feed back the output power source voltage of the second DC power source 28 to the switching control portion 16. According, the switching control unit 16 varies duty of PWM signal according, to the state of feedback voltage, thereby stabilizing the output power source.
And, a current sensing circuit 20 senses the state of the primary side current flowing through the switching unit 18 and supplies the sensed current state to the switching control unit 16. At this time, if current over a regulated value is sensed due to abnormality of input power source or the abnormality of load or SMPS, the switching control unit 16 is shut down, thereby stopping the generation of PWM signal to protect load or SMPS from overcurrent.
FIG. 4 is a diagram of a conventional current sensing circuit for sensing a current state of primary side as described above, where a switching control unit 16, a switching unit 18, a current sensing circuit 20, and lines 101 to 104 correspond to the corresponding circuits of the FIG. 3, respectively. The switching control unit 16 is constituted by a PWM controller as described above. A field effect transistor (hereinafter referred to as FET) 36 of the switching unit 18 switches a primary-side power source in response to a PWM signal, such as that of FIG. 5A, outputted from an output terminal OUTPUT of the switching control unit 16. In the line 101 which is a primary side of transformer 14, a voltage waveform such as FIG. 5B is shown by the FET 36. Resistors 32 and 34 are coupled between the output terminal OUTPUT of the switching control unit 16 and a gate terminal of the FET 36, to properly set on/off time of FET 36. A resistor 42 of the current sensing circuit 20 is a current sensing resistor, which limits current flowing through the FET 36 and at the same time, generates a voltage corresponding to the amount of current in the line 103. The generated voltage shows a waveform such as FIG. 5C and is supplied to a current sensing terminal I.sub.SENSE of the switching control unit 16 through a resistor 40 as a current sensing voltage having the waveform such as FIG. 5D.
Generally, a shut-down voltage where the switching control unit 16 senses overcurrent and is shut down is set as 1 V, and accordingly, a current sensing resistance Rs according to a maximum current Ismax for sensing overcurrent is determined by the following equation (1): EQU Ismax.about.1.0V/Rs (1)
Thus, if a current sensing voltage supplied to the current sensing terminal I.sub.SENSE of the switching control unit 16 through the resistor 40 reaches 1 V as the current passing through the FET 36 increases, the switching control unit 16 is shut down. At this time, since power proportional to the current flowing through the FET 36 is consumed on the resistor 42, heat loss is generated. For instance, when a maximum current Ismax where the current sensing voltage becomes 1 V, is 15A, a duty D of PWM signal is 0.8, and a resistance R.sub.42 of resistor 42 is 65 m.OMEGA., the power consumption Pt is given by the following equation (2): ##EQU1##
That is, to obtain the current sensing voltage of 1V, loss of 11.7 W is generated. Accordingly, when large current such as 15A is sensed, excessive heat loss is generated, so that additional radiating processing is required. Also, there is a problem in that the efficiency of SMPS is deteriorated by the generation of heat loss. Also, the resistor 42 should be a resistor having rated dissipation which is sufficiently large with respect to the power of 11.7 W, so that there are problems of occupying large space and raising the cost.
Accordingly, another current sensing circuit constituted by a transformer (troidal core transformer) 44 for sensing current by a magnetic element instead of the resistor 42, as shown in FIG. 6 is used. In FIG. 6, the switching control unit 16, the switching unit 18, the current sensing circuit 20, and the lines 101 to 104 correspond to the corresponding circuits of FIGS. 3 and 4, respectively. And, the switching unit 18 switches the primary-side power source in response to a PWM signal, such as FIG. 7A, outputted from the switching control unit 16 as described above. Then, the voltage waveform such as FIG. 7B is shown in the line 101. A resistor 46 converts magnetic current induced in the secondary side of the transformer 44 into a voltage, which is generated with the waveform such as FIG. 7C in the line 105. The voltage of the line 105 appears in the line 106 without negative voltage, as shown in FIG. 7D, through a diode 48, and is supplied to the current sensing terminal I.sub.SENSE of the switching control unit 16 through the resistor 52 as a current sensing voltage having the waveform such as FIG. 7E. A resistor 50 stabilizes the voltage of the line 106.
In this case, the power proportional to the current flowing through the switching unit 18 is consumed also in the resistor 46, so that heat loss is generated. For instance, when a maximum magnetic current Imax where the current sensing voltage becomes 1 V, is 150 mA and a resistance R.sub.46 of resistor 46 is 15.OMEGA., the maximum voltage V.sub.105 in the line 105 is given by the following equation (3): EQU i V.sub.105 =150.times.10.sup.-3 .times.15=2.25V (3)
Thus, when the duty D of PWM signal is 0.8, the power consumption Pt dissipated in the resistor 46 is given by the following equation (4): ##EQU2##
That is, the loss of 270 mW which is greatly reduced compared with the circuit of FIG. 4 is generated to obtain the current sensing voltage of 1 V. However, according to the use of magnetic element, the following problems are generated. First, the number of steps in manufacturing of products is increased and the automatic insert machine cannot be used. Secondly, in the design of the magnetic element, saturation of magnetic core should be considered, so that its realization is difficult.
As described above, in SMPS, since the conventional current sensing circuit uses a resistor element or a magnetic element to sense current state, the above-mentioned problems are generated and also there is another problem of causing a false operation since noise level with respect to large current is transferred as a current sensing voltage, as it is, during the transfer of current sensing voltage.